Lateral Patellar Instability:: Diagnosis, Treatment Algorithm, Conservative Approaches, Surgical Techniques, Outcomes, Rehabilitation and Future Directions


Lateral patellar instability is a common knee disorder, particularly in children and adolescents. Its aetiology is multifactorial, and typically it is caused by a combination of soft tissue and osseous abnormalities, such as medial patellofemoral ligament (MPFL) incompetency, dysplasia, and patella alta. The annual risk of lateral patellar dislocation ranges from 5.8 to 77 per 100,000 persons, with girls aged 10 to 17 at the highest risk for instability. ,

Dislocations usually occur in active individuals during sport. The patient typically presents with a painful knee effusion and provides a clinical history of sensing a click at the time of injury. The majority of lateral patellar dislocations will spontaneously reduce; however, about 20% of patients require assisted reduction by extending the leg with a gentle medial directed force on the patella. Patients who experience lateral patellar instability are at increased risk for future instability events, with a reported average ipsilateral recurrence rate of 36% to 69%. , Risk factors that place patients at increased risk of recurrence include structural abnormalities, such as trochlear dysplasia, elevated tibial tubercle to trochlear groove (TT-TG) distance (>20 mm) and patella alta, and demographic factors such as age younger than 18 and female sex. , The consequences of patellofemoral instability on the development of patellofemoral arthritis are not benign, and natural history studies have suggested that almost 50% of patients who experience patellar instability will be diagnosed with patellofemoral arthritis within 25 years. ,

The management of patellofemoral instability is complex given its multifactorial aetiology. The majority of first-time dislocations are often treated conservatively, with surgery reserved for those with an osteochondral fragment. In contrast, patients with recurrent dislocation who present with chronic complaints of continued debilitating instability and potential anterior knee pain are often treated with surgical intervention. Before surgical intervention, patients undergo clinical and imaging evaluation, which often includes radiographs and magnetic resonance imaging (MRI) with a rotation profile to further evaluate alignment, chondral surfaces and soft tissue injuries. The surgical treatment of patellofemoral instability is complex and is often directed at correcting factors associated with increasing risk for future patellofemoral instability events.

This chapter details the diagnosis and treatment, both conservative and surgical, of lateral patellar instability, with an emphasis on the role of the MPFL. Specifically it explains the patient history, physical examination findings and imaging that assist in making an accurate diagnosis. The treatment algorithm, with an analysis of the role of conservative versus surgical treatment, is also discussed. Finally, the specific surgical technique of MPFL reconstruction with patient outcomes will be reviewed.


Patient History

A thorough clinical investigation, including both the history and physical examination, is essential when evaluating a patient with patellar instability. Presentation of patellar instability lies on a spectrum from minor subluxation events to acute traumatic dislocations. In the setting of an acute patella dislocation, patients may describe an audible ‘pop’ or sensation of the patella shifting. Typical mechanisms include a noncontact twisting force through the knee with a planted ipsilateral foot or a direct force sustained during contact sports to the medial knee causing lateral translation of the patella. In these situations the patella may spontaneously relocate with knee extension or require a formal reduction. Patients will present acutely with a large hemarthrosis, pain and decreased range of motion. Absence of hemarthrosis in a patient presenting with instability typically indicates either mild subluxation events, recurrent dislocation episodes or atraumatic dislocation due to a lack of osseous restraint or ligamentous laxity. Patients with recurrent instability often have a different presentation from acute first-time dislocators. They may describe persistent anterior knee pain, subjective feelings of subtle laxity or that their knee is ‘giving way’. Some patients may have never experienced a complete patellar dislocation; however, instead they may describe chronic subtle subluxation and apprehension. It is important for the clinician to determine whether pain is present in correlation to instability events, because pain and swelling outside of instability events may indicate an underlying trochlear or patellar chondral lesion or another concomitant intraarticular disorder that may need to be addressed.

The clinician must inquire about the number and frequency of prior patellar dislocations because the rate of recurrent instability and failed nonoperative management rises in patients who have sustained more than one dislocation event. Age, sex and overall skeletal maturity should be documented because they also play a major role in recurrence risk. Prior treatments, including bracing, physical therapy and surgical interventions, are important to inquire about, especially if these include a prior stabilisation or lateral release. A history of ligamentous laxity or joint dislocation in the patient or family member should also be explored. In addition, the presence or history of symptoms in the contralateral knee should be documented. Lastly, it is important to determine the patient’s overall functional status, including activity level, profession, athletic involvement and level of sport participation. Patient expectations, especially in the athletic population with regards to return to sport, play a major role in management of patellar instability and should be determined during initial evaluation.

Physical Examination

Development of an efficient, comprehensive and systematic examination aids the orthopaedic surgeons’ assessment of the patient with patellar instability. The classic algorithm of inspection, palpitation, range of motion and special tests holds true when examining the patellofemoral joint. Before examination, ensure that the patient changes into shorts and removes socks and shoes so that both knees and feet are exposed.

Inspection begins with a skin assessment for swelling, bruising, erythema and previous surgical scars. In the standing position, overall limb alignment is generalised into neutral, varus, or valgus position. Increased femoral anteversion or external tibial rotation should be noted. Alignment can be subjectively quantified through measurement of the Q-angle, which is the angle formed between a line from the anterior superior iliac spine (ASIS) to the patella and a line from the patella to the tibial tubercle. The Q-angle should be measured both with knees fully extended and flexed 30 degrees. Generally a larger Q-angle indicates a larger lateral force placed on the patella and thus places the patient at an increased risk of lateral patellar subluxation and instability. The height of the patella should be noted regarding the joint line to determine whether patella alta or baja is present. From the sagittal plane, recurvatum should be noted as this may be an indicator of overall generalised ligamentous laxity. Dynamic assessment is carried out in the form of gait analysis and single-leg squats. With the patient walking, evaluate for an antalgic or Trendelenburg gait, in addition to the presence of in-toeing. Single-leg squats evaluate for overall lower extremity and core strength. Specifically, pelvic drop may indicate hip abductor and core weakness, whereas an inability to control neutral knee position can indicate overall inadequate quadriceps strength.

Palpation can be carried out in the seated position. Any effusion of the knee should be noted, and comparison with the contralateral knee can be helpful. Palpation of the knee and patellofemoral joint should follow a systematic course. The entire extensor mechanism should be palpated and inspected, followed by the patellar body, medial and lateral articular facets and inferior and superior poles of the patella. The medial and lateral knee joint lines should be palpated for tenderness, as well as the medial and lateral retinaculum, specifically the MPFL. The knee should be ranged with anterior compression on the patella (‘patellar grind test’), and any crepitus or pain provocation should be noted. Patellar mobility is assessed through both tilt and medial and lateral translation. With the knee in full extension and at 20 degrees of flexion, the examiner tries to evert the patella both medially and laterally. Inability to evert the lateral patella upwards may indicate an overly tight lateral retinaculum, whereas increased tilt may be due to a prior lateral release. Patellar displacement is also tested in the same position that translation is measured and recorded in quadrants (one to four) based on the percentage of patellar width correlated to displacement. Limited medial translation of less than one quadrant may indicate lateral retinacular tightness, whereas lateral translation of three or more quadrants is suggestive of patellar hypermobility. The presence or absence of a firm endpoint is denoted as A or B respectively. During lateral translation, if the patient becomes apprehensive about an impending dislocation event, this is considered a positive apprehension test. Additionally, with lateral translational hypermobility the knee should be progressively flexed until the patella becomes stabilised within the osseous constraints of the trochlea. The degree of knee flexion at which this point occurs should be recorded using a moving apprehension test. The J-sign is performed with the patient’s knee flexed in the supine position; the knee is slowly extended looking for lateral translation of the patella as it unlocks from within the trochlea. This should be documented with the amount of translation in regards to patellar quadrants. An assessment of overall generalised ligamentous laxity should be measured and recorded through the Beighton criteria ( Table 30.1 ).

TABLE 30.1

Beighton Criteria for Joint Hypermobility

Left Right
Passive dorsiflexion of fifth metacarpophalangeal joint > 90 degrees 1 1
Thumb able to reach the ipsilateral volar forearm 1 1
Hyperextend elbow > 10 degrees 1 1
Hyperextend knee > 10 degrees 1 1
Palms able to touch the floor when bent over at the waist with legs locked out (no bending allowed) 1

Gross range of motion of the knee should be tested for any diminished flexion or extension. Hip internal and external rotation should be tested because this may detect underlying femoral torsion. A full ligamentous examination of the knee is critical to evaluate for any concomitant pathological condition. Lastly, all examination manoeuvres should be performed on the contralateral knee to assess for asymmetry.


Standard imaging plays an important role in the evaluation of the patient with patellar instability. Conventional radiographs should be obtained first, with a standard series of anteroposterior, Rosenberg posteroanterior (45 degrees of flexion), lateral and Merchant views. In addition, standing mechanical axis films should be obtained to assess overall limb alignment. Plain radiographs provide important information with regards to overall patellofemoral anatomy in addition to determining whether arthritis or loose bodies are present. For instance, lateral view radiographs provide information regarding patella height and trochlear dysplasia. Patella height can be measured through several different methods based on radiographic landmarks on the patella and proximal tibial joint surface. The Insall-Salvati ratio measures the overall ratio of the patellar tendon length to the length of the patella, with normal values between 1.1 and 1.2 and patella alta defined as greater than 1.2 ( Fig. 30.1 ). Other methods of measuring patella height include the Caton-Deschamps and Blackburne-Peel indexes and modified Insall-Salvati ratio ( Figs. 30.2 and 30.3 ). These differ from the traditional Insall-Salvati ratio by not using osseous boundaries for measurement of patella length and instead using the articular surface length of the patella. These have shown higher interobserver reliability than the traditional Insall-Salvati ratio and thus have become the preferred methods of measurement. ,

Fig. 30.1

The Insall-Salvati Index is calculated by dividing the length of the patella (line A) by the measurement of the length of the patella tendon (line B) on lateral radiograph.

Fig. 30.2

The Canton-Deschamps Index is calculated by dividing the length of the patellar articular surface (line A) by the distance from the lower pole of the patella to upper tibia (line B) on lateral radiograph.

Fig. 30.3

The Blackburn-Peel Index is calculated by dividing the distance from the lower patellar articular surface to a line drawn perpendicularly from the upper tibia (line B) by the patellar articular surface length (line A) on lateral radiograph.

Trochlear dysplasia can also be assessed on lateral radiographs; however, this requires perfect overlaying of the posterior aspects of the medial and lateral femoral condyles to accurately evaluate the anatomy of the trochlea ( Fig. 30.4 ). Dejour et al. were the first to describe the ‘crossing sign’, which represents a line visualised as the trochlear floor crossing anteriorly to a line visualised as the anterior femoral cortex seen on a perfect lateral film. This is pathognomonic for trochlear dysplasia with up to 96% of individuals with patellar instability demonstrating a positive crossover sign on lateral imaging. The more distal this crossing line is seen, the more severe the dysplasia. Trochlear prominence is seen on lateral imaging with extension of the trochlea floor anterior to the projection of the anterior femoral cortices. This contributes to an increased risk of patellar instability by decreasing the relative lateral wall height of the lateral aspect of the trochlear groove and thus offers less resistance to lateral translation of the patella. Specifically, a prominence greater than 4 mm is associated with patellar dysplasia and instability. Lastly, morphologically the presence of lateral trochlear convexity and medial trochlear hypoplasia can be seen on the lateral radiograph as a double contour sign and also predisposes to patellar instability. Dejour et al. described a classification system based on trochlear morphology of the patellofemoral joint using the crossover sign, supratrochlear prominence and the double contour line. , The Merchant view is useful to asses patellar tilt, subluxation and overall patella–trochlea relationship. The sulcus angle can be measured from the highest point on the condyles to the lowest point within the trochlear sulcus. An angle greater than 145 degrees is associated with trochlear dysplasia. With higher knee flexion angles, more inferior portions of the trochlea are visualised, whereas the more commonly dysplastic proximal trochlea is not visualised. Thus up to 35% of trochlear dysplasia can go unrecognised on axial radiographic imaging. Also assessed on the axial radiograph is patellar tilt through the lateral patellofemoral angle. This is formed by a line along the anterior femoral condyles and a line along the lateral patellar facet ( Fig. 30.5 ). Normally this angle is greater than 8 degrees; however, the relative importance of this view has diminished with now more readily available advanced axial imaging modalities.

Fig. 30.4

An example of high-grade trochlear dysplasia on an axial T2 magnetic resonance imaging (MRI; right knee).

Fig. 30.5

Lateral patellofemoral angle measured on a Merchant view radiograph (A) . This angle is the intersection created by drawing a line connecting the anterior aspects of the femoral condyles and a line along the patellar lateral surface.

Advanced imaging in the form of computed tomography (CT) and MRI provides three-dimensional (3D) information of the patellofemoral joint, which can be beneficial in the evaluation of patients with patellar instability and should be ordered in patients with hemarthrosis because bone fragments can be missed when using only radiographs. MRI provides superior resolution of soft tissue structures within the knee. Specifically, axial T1 and T2 images can be used to evaluate the chondral surfaces of the patellofemoral joint for articular surface defects, which can be present in both acute and chronic patellar dislocations. MRI findings after acute patellar dislocations typically reveal a large hemarthrosis and the classic articular cartilage defect along the medial patellar facet with associated bony oedema involving the lateral femoral condyle that occurs during relocation impact ( Fig. 30.6 ). In addition, loose bodies may be seen within the suprapatellar pouch. MRI is also crucial in further identifying the extent of injury to the medial soft tissue restraints, including the MPFL, which is commonly attenuated or ruptured in the setting of patellar instability. Finally, axial MRI images also further define trochlear anatomy, including patellar tilt and dysplasia. CT imaging provides superior detail regarding osseous structures. Specifically, CT can be used to assess patellar tilt with the leg fully extended, which is the measured angle of the intersection between the width of the patella and the line connecting the posterior femoral condyles. Angles greater than 20 degrees suggest abnormal patellar tilt. Rotational alignment of the lower extremity can be assessed through specific CT version studies looking at both femoral and tibial version, which may contribute to patellar instability. Finally, both axial MRI and CT imaging can be used to measure the TT-TG distance or tibial tubercle to posterior cruciate ligament (TT-PCL) distance, which both provide a measure of tibial tubercle alignment and overall lateral offset ( Figs. 30.7 and 30.8 ). A TT-TG distance of more than 20 mm has been shown to be highly correlated (>90%) with patellar instability. As the distance increases, the tibial tubercle becomes more lateralised, creating a theoretically higher risk for lateral patellar instability, which may be an indication for distal surgical realignment procedures. TT-PCL is relatively new within the literature and is measured using axial MRI images. TT-PCL distance has the advantage over TT-TG distance in that it is not affected by femoral anatomy and rotation and thus may be a more reliable way to assess isolated lateralisation of the tibial tubercle. Seitlinger et al. propose a distance greater than 24 mm to be considered an abnormal tibial tubercle offset.

Fig. 30.6

Patella dislocation demonstrating bone oedema pattern (A, arrow ) and loose bodies (B, arrow ).

Fig. 30.7

Magnetic resonance imaging (MRI) tibial tubercle–trochlear groove (TT-TG) distance at (A) normal (TT-TG = 12 mm) and (B) abnormal (TT-TG = 28 mm) values on axial T2 MRI.

Fig. 30.8

Magnetic resonance imaging (MRI) tibial tubercle–posterior cruciate ligament (TT-PCL) distance at (A) normal (TT-PCL = 24 mm) and (B) abnormal (TT-PCL = 37 mm) values on axial T2 MRI.

Patella alta can also be calculated using MRI imaging. Interclass correlation coefficients have been shown to be higher on MRI than radiographs in both first-time patellar dislocators and controls when using the Blackburne-Peel and Caton-Deschamps indexes. In addition, the overlap of the patellar articular overlap and percentage of patellar articular on sagittal MRI can be used as a surrogate for patella alta measurement and have been shown to correlate with Blackburne-Peel and Caton-Deschamps indexes but not the Insall-Salvati ratio.


The treatment of patellar instability requires the separation of acute versus recurrent conditions because these subsets have different treatment methods. Conservative management is the treatment of choice for patients who experience an acute first-time lateral patellar dislocation without evidence of a loose body on imaging. This includes mobilisation, bracing or taping and therapy with early motion. Surgery is generally performed for patients who fail conservative management or patients with recurrent instability.

When preparing for surgical intervention, all the patient’s risk factors for subsequent instability or MPFL reconstruction failure should be taken into consideration. These factors include global factors such as tibial torsion, femoral anteversion, presence of femoral anteversion (>30 degrees), tibial torsion (>20 degrees), valgus (>15 degrees) and local factors such as patella alta (Caton-Deschamps Index (CDI) > 1.2), dysplasia (high-grade Dejour classification, dome trochlea or a large spur (>5 mm), lateral patellofemoral ligament (LPFL) deficiency and lateral tuberosity abnormalities (TT-TG > 17 to 20, TT-PCL > 20 to 24). Unfortunately, there is a lack of evidence for guidance on the effect of multiple risk factors on outcomes and recurrent instability. However, although all these factors should be measured preoperatively, they often do not need to be addressed in an initial MPFL reconstruction but should be considered to understand the surgical treatment if a failure occurs and a revision becomes necessary. For example, in the revision setting, patellar malalignment, as indicated by abnormal patellar height, patellar tilt, TT-TG and TT-PCL, may necessitate a tibial tubercle osteotomy (TTO) in conjunction with a revision MPFL reconstruction. In contrast, a femoral derotational osteotomy may be used in increased femoral anteversion (>30 degrees), whereas trochleoplasty or bumpectomy may be used in patients with significant trochlear dysplasia.

The remainder of this chapter focuses on the specifics of conservative and surgical interventions and the outcomes associated with treatment.

Conservative Treatment

As discussed previously, conservative treatment is generally reserved for first-time dislocators, especially those who are skeletally mature. Two of the most commonly implemented conservative measures include functional mobilisation with bracing or application of patellar tape and use of a brace. Bracing has been a commonly implanted conservative treatment for patellar instability and subluxation. Evidence has demonstrated that duration of brace treatment does not affect the rate of recurrent patellar instability. Immobilisation should be avoided because it has been associated with an increased risk of patellar redislocation.

The benefits of conservative treatment include the avoidance of surgical complications. However, reports in the literature have begun to question the role of conservative treatment given the exceedingly high recurrence rates. In addition, conservative treatment is not always successful in meeting patient expectations. For example, one study demonstrated that only 26.4% of first-time dislocators were able return to sport after conservative management. Future investigation is necessary to determine when conservative treatment should be pursued.

Surgical Technique

Medial patellofemoral ligament reconstruction technique

The patient is positioned supine on a regular table in the operating theatre with the end of the table dropped so the leg hangs in a position of 90 degrees of flexion. After anaesthesia and sterile prepping with chlorhexidine, a tourniquet is placed on the operative leg and the nonoperative leg is placed in the lithotomy position. An examination under anaesthesia is conducted to evaluate the degree of flexion at which lateral patellar dislocation occurs and well as hip internal rotation, hip external rotation and medial and lateral translation. Next a diagnostic arthroscopy is conducted to evaluate for signs of dysplasia, loose bodies and any chondral defects. After the arthroscopy, attention is turned towards MPFL reconstruction. A longitudinal incision is made at the superomedial border of the patella. Dissection is carried out to the osseous attachment of the MPFL on the patella ( Fig. 30.9 ), while the graft is then prepared with a whipstitch on both ends ( Fig. 30.10 ). A trough is created in the superior medial aspect of the patella with a rongeur involving the proximal 50% of the patella. Anchors are then inserted at the proximal-medial corner of the patella and at the equator of the patella ( Fig. 30.11 ). The placement is determined superficial to the capsule and medial landmarks are palpated. After anchor insertion, attention is turned to creating the femoral socket. A true lateral is obtained using fluoroscopy to identify the Schottle point. A 3-cm incision is then incised in the skin over this area. The tunnel is then connected so that the pathway of the MPFL can be palpated. A guide pin is placed at the Schottle point, after which the adductor tubercle is palpated just proximal to this point ( Fig. 30.12 ). Length changes are then checked with the sutures from the anchor being placed on the pin and changed with a hemostat. If the length changes are acceptable, the tunnel is reamed, typically with a 7- to 8-mm reamer. Using a Mason-Allen configuration with a free needle, the loop end is sutured into the trough area between the two anchors. The free ends are then brought through the femoral tunnel and fixated with a PEEK (polyether-ether-ketone) interference screw ( Figs. 30.13 and 30.14 ). The length is set in full extension to allow the graft to find its proper tension as it is taken through a range of motion. Arthroscopic examination is then used to confirm the appropriate extra capsular positioning of the graft. Translation after fixation should be one to two quadrants without over constraint.

Fig. 30.9

Identifying the insufficient medial patellofemoral ligament (MPFL; white arrow).

May 3, 2021 | Posted by in ORTHOPEDIC | Comments Off on Lateral Patellar Instability:: Diagnosis, Treatment Algorithm, Conservative Approaches, Surgical Techniques, Outcomes, Rehabilitation and Future Directions
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